1. Field of the Invention
The present invention relates to an ink for ink-jet recording and a color ink set composed of inks having a plurality of colors, and to an ink-jet recording apparatus.
2. Description of the Related Art
In the ink-jet recording method, ink droplets are formed by means of a variety of ink discharging process including, for example, the electrostatic attraction process, a process in which mechanical vibration or displacement is applied to the ink with a piezoelectric element, and a process in which bubbles are generated by heating the ink to utilize the pressure generated thereby, and all or a part of the ink droplets are adhered to a recording objective such as paper to perform the recording.
The ink, which is used for the ink-jet recording method as described above, includes those in which various water-soluble dyes or pigments are used as color materials, and they are dissolved or dispersed in a liquid medium composed of water or water and a water-soluble organic solvent. The dye and the pigment may be compared with each other as follows. That is, the dye is inferior in water resistance because of such a property that the dye is completely dissolved in the ink medium, and the dye is also inferior in light resistance to a considerable degree due to its chemical structure. On the other hand, the coloration of the pigment is not so vivid as compared with the dye, but the pigment is extremely excellent in weather resistance including water resistance, because the pigment is water-insoluble. Therefore, in recent years, the changeover is progressively effected for the ink for ink-jet printer from the ink containing the color material of the dye to the ink containing the color material of the pigment.
In the case of the ink composed the water-based medium containing the color material of the pigment, it is necessary that the pigment, which is insoluble in the water-based or aqueous medium, is finely dispersed in a stable manner for a long period. Those suggested as the method for dispersing the pigment in the water-based medium include a method described in U.S. Pat. No. 5,571,311 which uses a self-dispersing pigment applied with a surface treatment, for example, such that a hydrophilic group is introduced into the pigment surface, and a method in which a dispersing agent such as a surfactant and a polymer dispersing agent is added to a liquid medium.
The effect of the surface treatment is restricted by the chemical property of the pigment itself. Therefore, all of the pigments cannot be dispersed in the water-based medium in an effective manner. Therefore, the type of the pigment, which can be used as the self-dispersing pigment, is limited. Further, the color development of the pigment is inhibited, and neither sufficient optical concentration nor desired color is not obtained in some cases, because the pigment surface is chemically treated. The color ink, which contains the color material of the self-dispersing pigment, sometimes inferior in coloration as compared with the ink which contains the color material of the dye.
On the other hand, in the case of the ink in which the pigment is dispersed in the water-based liquid medium by using the dispersing agent, it is possible to obtain the ink having satisfactory coloration as compared with the ink based on the use of the self-dispersing pigment, because the pigment can be widely selected, and the pigment surface is not chemically treated.
Those usable as the dispersing agent include, for example, anionic surfactant, nonionic surfactant, cationic surfactant, and water-soluble polymer. Especially, a variety of anionic surfactants have great effects as the dispersing agent, with which the pigment is stably dispersed. However, in the case of the ink which uses the anionic surfactant as the dispersing agent, any excessive anionic surfactant, which does not principally contributes to the dispersion, considerably lowers the surface tension of the water-based liquid medium, and thus the permeating action of the ink into the paper is conspicuously facilitated. For this reason, when the ink droplets, which contain the anionic surfactant, are adhered to the recording paper, then the ink droplets quickly permeate into the paper, and they also permeate non-uniformly in the directions parallel to the paper surface. As a result, any blurring tends to occur, and it is impossible to obtain a sharp image. Further, for example, the following problem has also arisen. That is, the pigment scarcely stays on the paper surface, and the printing concentration becomes thin, because the ink excessively permeates into the paper. As for the blurring as described above, the whisker-like non-uniform blurring, which appears at portions subjected to the printing with a single color, is called xe2x80x9cfeatheringxe2x80x9d. The non-uniform blurring, which is caused by the inks having different colors mixed with each other at adjoining portions, is called xe2x80x9cbleedingxe2x80x9d.
Recently, in the offices, the combined information, which is composed of, for example, text letters and graphs, is recorded in many cases. When the ink-jet printer is used in such an office, it is especially demanded that the feathering and the bleeding scarcely occur, for example, on letters and ruled lines, and the printing concentration is high.
The present invention has been made in order to solve the problems as described above, an object of which is to provide an ink for ink-jet recording and a color ink set with which it is possible to obtain a sharp image having little feathering and little bleeding with a high printing concentration even when an anionic surfactant is used as a dispersing agent for a pigment. Another object of the present invention is to provide an ink-jet recording apparatus which accommodates the ink set of the present invention.
According to a first aspect of the present invention, there is provided an ink for ink-jet recording, comprising a pigment; an anionic surfactant; and a cationic surfactant. In the ink of the present invention, the anionic surfactant is principally used as a dispersing agent for the pigment. It is feared that some molecules of the anionic surfactant, which do not contribute to the dispersion of the pigment, i.e., free anionic surfactant molecules may cause the excessive permeation of the ink into the paper, and any feathering may be caused thereby. The cationic surfactant molecules are added for the free anionic surfactant molecules, and thus it is possible to suppress the excessive permeating action of the anionic surfactant. Accordingly, the content of the cationic surfactant in the ink is smaller than the content of the anionic surfactant, which is usually not more than xc2xd of the content of the anionic surfactant. In order to compensate the quantitative balance between the anionic surfactant and the cationic surfactant, the ink of the present invention may further comprise an amino acid having an amino group of positive polarity and a carboxyl group of negative polarity. The ink of the present invention may further comprise a glycol ether as a permeating agent.
According to a second aspect of the present invention, there is provided a color ink set for ink-jet recording, comprising a first ink which contains a first pigment, a glycol ether, an anionic surfactant, and a cationic surfactant; and a second ink which contains a second pigment different from the first pigment, a glycol ether, an anionic surfactant, and a cationic surfactant. When the ink set is used, it is possible to effectively avoid the bleeding of the first ink and the second ink subjected to the printing. The ink set may further comprise third and fourth inks having mutually different colors.
According to a third aspect of the present invention, there is provided an ink-jet recording apparatus comprising an ink-jet head; an ink tank which accommodates an ink to be supplied to the ink-jet head; and an ink set of the present invention which is accommodated in the ink tank. The ink tank may be an ink container fixedly provided in the apparatus or an ink cartridge which is replaceable.